Reversing servo for fuel control



Feb., 26, 1957 s- G, BEST REVERSING SERVO FOR FUEL CONTROL 4Sheets-Sheet 1 Original Filed Aug. 3l, 1951 Feb, 26, 1957 s. G. BEST2,782,767

REVERSING SERVO FOR FUEL CONTROL n [W2/esa Zow- Feb.1 26, 1957 s. G.BEST 2,782,767

REVERSING SERVO FOR FUEL CONTROL Original Filed Aug. 31, 1951 4Sheets-Sheet 3 7' E MPE RH TURF SENS/NG I//V/ 7' Feb. 26, 1957 s. G.BEST 2,782,757

REVERSING SERVO FOR FUEL CONTROL Original Filed Aug. 5l. 1951 4Sheets-Sheet 4 REVERSING SERV() FOR FUEL CONTROL Stanley G. Best,Manchester, Conn., assignor to United Aircraft Corporation, EastHartford, Conn., a corporation of Delaware 3 Claims. (Cl. 121-41) Thisapplication is a division of application Serial No. 244,551, led August31, 1951.

This invention relates to servo systems and more particularly to servosystems for fuel controls and the like.

it is an object of this invention to provide a servo mechanism formoving a throttle valve member or the like which mechanism can provide areversing motion upon movement of the control valve a predetermineddistance in a given direction.

This and other objects of this invention will become readily apparentfrom the following detailed description of the drawings in which: I

Fig. 1 is a schematic illustration of a turbo power plant with aschematically indicated fuel control operatively connected thereto.

Fig. 2 is a diagrammatic illustration of the fuel control of thisinvention arranged for a turbo-jet power plant.

Fig. 3 is a detailed illustration of the compressor pressure servomechanism.

Fig. 4 is a diagrammatic showingk illustrating the fuel control of thisinvention as modified for a turbo-prop power plant.

Fig. 5 is a schematic showing of a temperature compensating mechanismfor the fuel control.

Fig. 6 is a schematic showing of an additional feature for producingisochronous speed governing.-

Referring to Fig. l, a gas lturbine power plant is generally indicatedat 10 and the power plant includes an air inlet 12, a compressor 14, acombustion section 16, a turbine section 18 and an exhaust nozzle 20.The turbine portion of the power plant is arranged to drive thecompressor alone in the case of a turbo-jet power plant and to alsodrive a propeller 22 in the case of a turboprop power plant.

Where the power plant is of the turbo-prop type the usual variable pitchblades may be used to'be controlled by a speed governor which can be setby a pilots lever as schematically illustrated. For normal operation thepropeller would have a low pitch stop as is well-known in the art.

' VThe fuel control 26 senses the values of the same parameters of powerplant operation whether utilized with a turbo-jet or a turbo-prop. Asillustrated in Fig. 1, the fuel control senses R. P. M. of the powerplant, compressor inlet temperature and compressor outlet pressure. Apilots control lever 30 is operatively connected to the fuel control 26for proper regulation thereof while fuel under pressure enters the fuelcontrol and from there is fed to the fuel nozzles at the combustionsection of the power plant.

Fig. 2 diagrammatically illustrates the fuel control of this inventionfor a turbo-jet power plant. Fundamentally this fuel control providestwo signals which correspond to compressor pressure and speed,respectively, and multiplies these signals through linkages to positiona throttle valve. A constant pressure drop is maintained -across thethrottle valve so that each position of the' valve corre- 2,782,767Patented Feb. 26, i957 sponds to a definite fuel ow. Signalscorresponding to engme speed and compressor inlet temperature are fedthrough a three dimensional cam to override the normal control and Varythrottle valve opening in mixed relation w1th compressor pressure tolimit fuel flow accordingly.

The use of compressor pressure as a controlling parameter has severaldistinct advantages over the use, for example, of inlet pressure to thepower plant.

First there is better protection against battle damage. lf a bulle-thole is made in the compressor so air is bled of in large quantity, thecompressor pressure falls olf and cuts back fuel flow, preventingoverheating of the engine. With a control based on inlet pressure, thecontrol would not recognize the damage and would continue feeding infuel at the same rate as before, which would overheat the engine becauseof the reduction in air flow through the burners. Second, along the sameline, the temperature limiting is still very nearly correct when air isbled olf the compressor for operating accessories, whereas it would notbe in the case of a control based on inlet pressure. Third, greaterconsistency can be expected, since absolute pressure at the turbinenozzles, which is substantially equal to compressor discharge pressure,is, at a given limiting turbine inlet temperature and normal chokednozzle conditions, the single factor which directly determines mass airflow through the engine. Inlet pressure determines mass llow in a muchmore indirect manner. Fourth, the effects of variations in speed andtemperature are reduced radically so that accurate measurements of thesequantities are not required and design of the three dimensional cam isgreatly simpliied.

Referring to Fig. 2, a low pressure pump 50 and a high pressure pump 52may be provided as well as a relief valve 54 for the low pressure stageand a relief valve 56 for the high pressure stage. A filter 60 isprovided prior to the entry of the fuel into the system and this filtermay include a spring 62 to permit by-pass of the fuel when apredetermined pressure across the filter obtains. In the event of lilterclogging the pressure will raise the filter 60 against spring 62 so thatfuel will flow around the filter. High pressure fuel enters the line 66to the inlet chamber 63 of a throttle valve generally indicated at 7G.As the throttle valve is opened fuel passes through the chamber 6i; to4the chamber 72'and then to the lin-e 74 leading to the nozzles of thepower plant. A T in the line 66 leads to a pressure regulator valve76which includes a chamber 78 and a chamber 80 separated by a diaphragm82. The chamber Sil of the pressure regulator valve 76 communicates viaa line 84 with the outlet side of the throttle valve, i. e., the chamber72. The chamber 78 is under pressure equivalent to the inlet pressure tothe throttle valve. The spring 86 biases the valve toward a closedposition and the relative strength of this spring and the forceresponsive areas 4at opposite ends of the pressure regulator valve 76determine the xed pressure drop which will be maintained across thethrottle valve. Any fuel that is by-passed by the pressure regulatorvalve is carried back to the inlet of the pump 52 via the line S8. ltwill of course be understood that the throttle valve 70 will have xedmaximum and minimum opening stops incorporated therein.

The throttle valve 70 is biased toward a closed position by a spring 90which bears against an enlarged upper portion 92 of the movable elementof the valve. Motion is imparted to the movable valve element by meansof a pivoted arm which has its free end engaging a movable knife edge152. interposed between the arm :lull and the valve head 92 is a roller104 which is carried by a reciprocating rod 106 which in turn is movedby a piston 110 of the main servomotor. The mechanism consisting of thearm 100, the roller 104 and the knife-edge V102 is arranged to multiplythe motion of the knife edge 102 and the roller 104 as reciprocated bythe rod 106.

The knife edge 102 moves in a predetermined direction linearly withcompressor pressure but after a predetermined pressure has been reachedthe knife edge 102 moves in the opposite direction linearly withcontinued increase in compressor pressure to provide a maximum pressurelimiter by reducing fuel flow after the preselected pressure has beenreached. The mechanism providing this type of movement wil be describedimmediately hereinafter.

Compressor discharge pressure is admitted internally of a bellows 120which has its free end engaging an arm 122 which is fixed to and pivotsabout the axis of a rod 124. A second bellows 126 is evacuated andactsin opposition to the bellows 120 thereby providing motion to the arm122 as a function of absolute pressure in the compressor. Motion of thearm 122 is transmitted via the rod 124 to another arm 128 which engagesa pivoted member 130. The arm 130 in turn transmits motion to thecompressor pressure servo system generally indicated in Fig. 2 as 134and illustrated in better detail in Fig. 3.

As was previously mentioned, the compressor pressure servo mechanism 134and its knife edge 102 operates so as to increase or decrease throttlevalve setting by increasing or decreasing its effect on the multiplyinglinkage. Referring to both Figs. 2 and 3, the operation of thecompressor pressure servo system is best described in the followingmanner. As compressor pressure increases the arm 122 and likewise thearm 128 will be urged in a downward direction. This causes downwardmovement of a movable valve element 140 so that the port 142 willcommunicate with the port 144 so that high pressure fuel from thechamber 146 will ow to the chamber 148 adjacent the top of the movablevalve element 140. High pressure in the chamber 148 causes the servomain body 150 to move downward to again close off communication betweenthe ports 142 and 144. In the event that this downward motion of thevalve element 140 and the main body 150 and, of course, the knife edge102 is continued or repeated, a land 154 carried by a central xed valveelement will cover the port 156 so that any further movement of themovable valve element 140 in a downward direction will cause port 156 tobe placed in communication with port 158 via the annulus 160 so thatfuel in chamber 148 will be connected to the line 164 leading to thedrain line 166. When the chamber 148 is subjected to drain pressure thehigher pressure in chamber 146 will tend to move the main body portion150 and its accompanying knife edge 102 in an upward direction in directresponse to continued increase in compressor pressure. This in turncauses a decrease in fuel flow by closing the throttle valve.

Normal control of the roller 104, the rod 106 and the servo piston 110is determined by a speed signal which is obtained'from a lyball speedgovernor 180. The governor is driven by a drive shaft and, dependingupon the off-speed condition, the governor positions a pilot valve 182.High pressure fuel is admitted to the valve 182 via a line 184 while lowpressure fuel or drain pressure exists in the chamber 186 within thebody of the governor 180. It is then apparent that movement of the valve182 to the left will cause high pressure uid to flow in the line 188through the port 190 of a second pilot valve 192 thence to the annulus194 and line 196 and then to chamber 198 to move the piston 110 towardthe left. Under these conditions, although both ends of the piston 110are subjected to high pressure, the area of the right-hand side of thepiston 110 is larger than the left-hand side; therefore, motion to theleft results. Motion of the piston 110 to the left causes a decrease infuel flow through the throttle valve. The decrease in fuel results fromthe fact that the lever 100 is just below a horizontal position for zerocompressor pressure and sloping down toward the right at other values ofcompressor pressure. Under such contemperature responsive bellows.

ditions movement of rod 106 to the left will cause the throttle valve tomove toward a closed position. In the event that an underspeed conditionexists, the yweights of the governor 180 will move the pilot valve 182to the right thereby directing drain pressure through the line 188 andeventually to the line 196 in the chamber 198 so that the high pressureon the left-hand side of the main servo piston will move the piston tothe right to increase its effect on the multiplying linkage and thus tomove the valve for increased fuel flow. It will be noted that the speedgovernor includes a spring 200 which has an operative connection via thelever 202 to the main servo piston 110. Thus, each movement of the servopiston 110 causes a resetting of the governor 180 thereby providing apermanent droop in the response of this governor. A second spring 204 isadjusted via a cam schematically illustrated at 206 which is moved inrespouse to movements of the pilots lever 208. Hence, different powersettings will correspond to different speed settings of the speedgovernor 180. With a droop governor of the type shown a signal isobtained which levels out at a predetermined high speed to establish inthe throttle valve a minimum fuel flow which is proportional tocompressor pressure.

It will be noted that the pilot valve 182 controlled by the speedgovernor 180 is capable of controlling the main servo piston 110 onlywhen the valve 192 is positioned as shown in Fig. 2. The pilot valve 192acts as an overriding mechanism which responds to a function excessivetemperature at the compressor inlet and the speed of the power plant.The valve 192 then is a maximum limiting valve. Motion is imparted tothe movable member 220 of the valve 192 by means of a lever 222 and aknife edge 224. Motion is imparted to the knife edge 224 by a threedimensional cam 226 which rotates in response to compressor inlettemperature and reciprocates in response to power plant speed. Theoperational movements of the cam in response to these parameters will bedescribed hereinafter. However, when the desired limits are reached theknife edge 224 is moved to the right as is the central valve element 220of the valve 192. This motion of the valve element 220 places the line196 in communication with the high pressure fuel line 230 so that highpressure fuel exists in the chamber 198 on the right-hand side of theservo system 110 to cause a corresponding decrease in fuel ow.

It should be added that the minimum opening of the throttle valve 70 isset by an adjusting screw 212 which engages the right-hand end of themain servo piston 110 to limit the pistons leftward movement.

In order to obtain rotation of the three dimensional cam 226 in responseto compressor'inlet temperature, a bulb 250 is provided whichcommunicates with bellows 252. Variations in pressure within thesebellows causes motion through a rack and pinion 256 which in turn movesa pivoted arm 258. The bellows 254 acts as a compensating bellows whilethe bellows 252 is the The compensating bellows 254 is connected to line253 which is closed at its terminus adjacent the temperature sensingbulb. Hence any variations in temperature which may occur along thelines between the sensing bulb and the bellows will be compensated forat the bellows. In other words, any variations occurring between thesensing bulb and bellows 252 will be counteracted by similar butopposing action by bellows 254 on pinion 256. Motion of the arm 258moves the valve element 260 so that downward motion of the valve willconnect the chamber 262 with drain pressure then existing in the chamber264. Upward motion of the valve 260 connects the chamber 262 with thehigh pressure chamber 266. With high pressure in the chamber 262 themain servo body 268 will be moved upwardly so as to impart motion to therack and pinion 270 and also the gears 272, 274. The gear 274`is splinedat 276 with the main cam body so as to rotate the camina givendirection. 'When drain pressure exists in the chamber 262 the Ihigh1pressure'in the chamber '266 'forces the Vservo main body 268 in a-downward direction to rotate the three dimensional cam 226 intheopposite direction.

The three dimensional cam 226 is reciprocated by means of a speed4responsive servo system which provides a permanent droop .in thegoverning action of the system. A speed governor 290 is driven by adrive shaft 292 and acts to reciprocate a pilot valve '294. The pilotvalve 294 permits communication of high pressure from the line 296 ordrain pressure from the line 29S into the line 309 which leads to theupperchamber'302 and the lower chamber 304 at opposite Vends of 'thepilot valve. This insures equal force lon either end of the pilot valve.However, at the same vtime the Icam -226 is forced in either lof twoldirections depending onWhether drain pressure or highpressure is lbeingadmitted tothe chamber 302 adjacent thelowerend yofthe cam body. It will.be noted that with each :axial movement Iof .the cam 226 the tension on:spring :310 willbe varied :so that `inreality the Aspeed governor 290will 'be reset. This provides 'a servo mechanism in which the cam 22.6will 'have a definite position for each speed setting ofthe governor.

The surface conto-ur fof the `cam .226 isdeterminedby superimposing afamily of curves corresponding to the compressor surge line and maximum,desirable temperature. The curves in the final analysis are plotted as`a ycomparison of the ratio of .fuel .flow and compressor pressureagainst engine speed. The family of curves is obtained by plotting `themfor various values of linlet temperature. The surge line and .maximumdesired temperature together then define the cam contour. The

theoretical derivation of .these curvesis omitted herein forconvenience. is that point at which under particular conditions thecompressor operates errati-cally.

From the foregoing `description it is evident that the major controllingparameters are that of engine speed and compressor pressure. Signalsequivalent to these parameters are multiplied for controlling a singlethrottle valve. The speed controlling signal is subject to beingoverridden =by a maximum limiting signal corresponding to speed andcompressor inlet temperature. The compressor pressure signal is subjectto vbeing modified or limited inits controlling effect by the reversingmechanism in the compressor pressure servo system 134.

The governor 180 of Fig. 2 and its related system may include atemperature compensating 'device which is schematically illustrated inFig. 5. Thus a speed governor 18)b is shown including a reset spring 330which is varied in tension `upon movement of a lever 332 about its pivot334. The main servo which feeds motion to the multiplying mechanism atthe throttle valve tends to reset the governor for each positionthereof. For variations in ambient air temperature certain compensationmay be desired either to maintain the pow-er plant speed constant atgiven power settings orto vary speed according to a desired schedulewhile operating through varied ambient air temperatures. To this end acam 336 may be utilized to vary the position ofthe pivot 334 in responseto variations in ambient air temperature. The particular type of desiredresponse will be determined by the profile of cam 336.

In addition, it may be desirable to avoid a permanent droop effect inthe governor 180 of Fig. 2 and its related system. To obtain this effecta dashpot may be provided to produce a temporary feedback to the resetspring upon movement of the main servo. This additional feature is Ibestshown in Fig. 6. Here the governor has a setting spring and a preloadspring 342. The pivot p-oint 344 has a dashpot connected thereto, asillustrated. Thus, movements of the main servo will have only atemporary effect on the setting of the governor and provides isochronousgoverning action.

lt should be added ,that compressor surge f f6 'Referring to Fig. 4, `aturbo-prop version of the fuel control of this invention is illustrated.The Fig. "4 fuel system is substantially identical to'that .illustratedin Fig.

2 with a few structural elements added thereto. Hence, in describingFig, 4,.repetiticn of the operation of several major components will beomitted for convenience.

Thus the Athrottle valve 7@ is identical to the Fig. 2 construction andis .operated in substantially the same manner. The .compressor pressureservo lsystem 134a moves its knife edge .i102 to increase fuel `flowproportional to `compressor' pressure. Though not 'shown herein, asystem identical :to 134 -of Fig. 2 may beused so that 'the servo systemoperates to :move the knife edge to increase fuel in response topressureincrease up to a predetermined pressure and .then to decreasefuel ow with further -increase in compressor pressure. The servo system134e: operates to lmove knife edge 102 in response to movements lofvalve 320 which directs either high or low pressure fuel to the chamber322 above the main servo body 324. Motion of the knife edge '102 ismultiplie-d by .the 4motion of the main servo piston 116. In the case ofthis construction, however, the speed governor 189e is utilized as anunderspeed governor rather vthan as a primary controlling unit.

Thenormal lor ,primary control utilizes a function of compressor-inlettemperature and power plant speed with the resultant motion produced bythe temperature-sensing servo andthe Vspeed governor 29M on the threedimensional cam 226g. Thus the cam226a includes anormal controlcam-surface7 a maximum limit cam surface and an overspeed ycam ysurfaceas labelled in Fig. 4. VWith the temperature sensing servo and the speedgovernor 290e,

.including its servo system, operating in the manner described inconnection with Fig. 2 the normal contro-l cam portion of the cam226aoperates a member 410 which im- .parts motion to a lever 412 via aroller 41.4. The roller vi varies the fulcrum between the member 410 andthe lever 412 by being reciprocated lby a rod 416. Rod 416 is in turnoperated through a bell crank 418 which has an operative connection tothe pilots lever 20S. The output of the lever 412 is in reality equal tothe lproduct of thernotion resulting from the speed and temperaturefunction and the motion of the roller 414 as caused by movement of thepilots lever 208. As dilfering now from the controlillustrated in Fig.2, the output of the lever 412 is imparted to the :movable portion 422of a normal control pilot valve 424 which is interposed in seriesbetween the pilot valve 182e of the speed governor 18th: and the maximumlimit pilot valve 192.

Since the speed governor la functions as an underspeed governor it willlhave a setting somewhat lower than the propeller lgovernor illustrated`in Fig. l. Then, as far as the governor o is concerned, the power plantin the normal range of operation is continuously overspeeding t-o theextent that high pressure fluid will be directed into the line 188awhich leads to the normal control pilot valve 424. The normal controlpilot valve 424 can permit this high pressure flow to enter the line 428leading to the maximum limit pilot valve 192 or else it `can admit drainor low pressure fluid via the line 43d to the line 428 leading to themaximum limit valve E92. In the normal control range then the pilotvalve 82a of the speed governor 18th: and the maximum limit pilot valve192 are not effective to control the flow of fluid leading to the mainservo via the line 196rz.

As previously described, the maximum limit pilot valve 192 in responseto excessive temperature or lspeed can permit high pressure iluid toflow from the line 298g into the line 196a to move the main servo piston11i) to the left to decrease fuel fow.

As stated above, the propeller governor will have a higher speed settingthan the underspeed governor 180a to the extent that the propellergovernor will, for eX- ample, be set to move the propeller bladesagainst their low pitch stop, for instance below 1000 R. P. M. The

' 206a, will be set to take effect for example at 950 R. P. M.

Thus at power plant speeds below 1000 R. P. M., as for example when thepower plant is throttled back in landing approach of the aircraft, thenormal control pilot valve will be positioned so as to permitcommunication of iluid from the pilot valve 182:1 via the line 188a downthrough the maximum limit pilot valve 192 and eventually to the mainservo and its piston 110. The normal control pilot valve 424 ispositioned in this manner in low power settings of the pilots lever 208since the bell crank 418, rod 416 and roller 414 will be positioned tomove the lever 412 and the movable element 422 of the pilot valve 424 tothe right to open the line 428 to line 188a leading from the pilot valve182a.

In a turbo-prop installation and particularly in multiengine operation,having an underspeed governor is of primary importance. At low powersettings and low or zero thrust, a turbo power plant is developing andconsuming power in the turbine (to drive the compressor) at a relativelyhigh rate. Therefore, Small variations in fuel ilow in this range canresult in rapid and large changes in positive or negative thrust output.In a multiengine installation considerable yaw in the aircraft wouldnormally be experienced. The underspeed governor then accuratelycontrols power plant speed and thrust by regulating fuel ow in thispower range setting to overcome sudden variations in thrust.

Another reason it is desired to have the underspeed governor controlpower plant speed inthe low prop R. P. M. range is that in the event ago-around is necessary, immediate response of the power plant isdesired. Since the inertia of a gas turbine power plant is relativelyhigh, it is desirable to maintain its speed relatively high in the lowpower settings. Thus the underspeed governor maintains a relatively highpower plant speed by directly controlling fuel flow in multipliedrelation with compressor pressure when the propeller blades of aturbo-prop installation are against their low pitch stop and the pilotslever is at a low power setting other than in the idle or olf position,for example, in a ight idle position. The underspeed governor alsocontrols the ground idle ofthe power plant.

It is, of course, apparent that the idle or minimum flow setting for thethrottle valve 70 is preset by the adjsting mechanism 212a as in thelcase of the fuel system illustrated in Fig. 2.

- It will be evident that as a result of this inventionfan accurate,'highly responsive but rugged fuel control has been provided which isadaptable yto various types of gas turbine power plants.

Although certain embodiments of this invention have been illustrated anddescribed herein, it is to be understood that various changes andmodifications may be made in the construction and arrangement of thevarious pants without departing from the scope of this novel concept. l

What it is desired to obtain by Letters Patent is:

l. In a servo mechanism having a member to be moved, said member havingtwo operative sides, a source of uid under pressure, means forcontinuously exposing one of said sides to fluid from said source, adrain, valve means movable for selectively connecting the other side ofsaid member to said source or drain to eiect movement of ysaid member ineither of two directions, and means for reversing -the movement of saidmember upon movement of said valve means beyond a predetermined positionin one of its directions of movement.

2. In a servo device, an apertured piston, a source of fluid underpressure, a valve relatively slidable in the aperture of said piston andregulating the flow of fluid to at least one side of said piston to movethe lat-ter, said piston being normally moved in one direction byrelative motion of said v alve in a given direction, and meanscooperating with said valve for moving said piston in another directionupon movement of said valve a predeter- References Cited in the tile ofthis patent UNITED STATES PATENTS 2,433,420 Booth Dec. 30, 1947

